Thermal energy storage systems which keep warm and cold water separated by means of gravitational
stratification have been found to be attractive in low and medium temperature thermal storage applications
due to their simplicity and low cost. This effect is known as thermal stratification, and has been
studied experimentally thoughtfully. This system stores sensible heat in water for short term applications.
Adding PCM (phase change material) modules at the top of the water tank would give the system
a higher storage density and compensate heat loss in the top layer because of the latent heat of PCM.
Tests were performed under real operating conditions in a complete solar heating system that was
constructed at the University of Lleida, Spain. In this work, new PCM-graphite compounds with optimized
thermal properties were used, such as 80:20 weight percent ratio mixtures of paraffin and stearic
acid (PS), paraffin and palmitic acid (PP), and stearic acid and myristic acid (SM). The solar domestic hot
water (SDHW) tank used in the experiments had a 150 L water capacity. Three modules with a cylindrical
geometry with an outer diameter of 0.176 m and a height of 0.315 m were used. In the cooling experiments,
the average tank water temperature dropped below the PCM melting temperature range in about
6–12 h. During reheating experiments, the PCM could increase the temperature of 14–36 L of water at the
upper part of the SDHW tank by 3–4 C. This effect took place in 10–15 min. It can be concluded that PS
gave the best results for thermal performance enhancement of the SDHW tank (74% efficiency).